Precipitation measurements from GPM are expected to improve our assessment of the global water cycle, enable better climate prediction, and enhance scientists' weather forecasting abilities. But GPM data is also likely to positively influence the study of one of nature's most dangerous weather events-the hurricane.

Scientists have already successfully utilized more than seven years of data from the spaceborne Tropical Rainfall Measuring Mission (TRMM) to aid in hurricane studies. The GPM era will be particularly exciting for hurricane scientists and forecasters because GPM will provide nearly global precipitation data almost every three hours-a vast improvement over the sporadic TRMM coverage. Scientists expect to use GPM data to achieve corresponding advances in hurricane research. In this article, we take a look at how scientists have used TRMM rainfall data to study hurricanes, and explore how GPM data may further hurricane research.

Monitoring Tropical Cyclones and Their Environment

One of the most serendipitous uses of TRMM data has been in monitoring the "fuel supply" for hurricanes. Hurricanes form and thrive in warm ocean waters exceeding 27 degrees Celsius. The passive microwave radiometer on TRMM (a similar instrument will fly on all of the GPM constellation satellites) provides a capability to measure SST even when clouds are present. The global SST map (Figure 1, below) derived from TRMM and another passive microwave radiometer, illustrates that the Atlantic Ocean and Gulf of Mexico waters were very warm during the summer of 2004-a banner year for hurricane development.

Figure 1: Global Sea Surface Temperature map derived from data from TRMM and another passive microwave radiometer

Traditional infrared (IR) techniques for measuring Sea Surface Temperature (SST) are only effective under clear sky conditions. In a typical hurricane environment, however, numerous clouds exist. In Figure 2, a cold wake (in blue) from Hurricane Bonnie (1998) is detected by the TRMM microwave imager. This cold wake resulted in the temporary weakening of Hurricane Danielle, which moved into the region a few days later.

Figure 2: Cold wake (in blue) from Hurrican Bonnie off the Florida coast (1998)

TRMM measurements also provide a good assessment of rainfall in the Western Sahel region of Africa. What does this have to do with hurricanes? Many Atlantic storms are born as easterly waves that come from the continent of Africa. Several scientific studies have shown correlations between active hurricane seasons and pre-hurricane season rainfall in western Africa.

Surprisingly, one of the most dangerous elements of tropical cyclones is the inland freshwater flooding that occurs during landfall. Scientists are experimenting with TRMM data in combination with infrared measurement techniques to produce approximate three-hour rainfall totals for areas affected by hurricanes. While this merged technique may contain serious errors, it is a useful "test drive" for the era when GPM will provide more accurate, all microwave-based rainfall estimates over a similar time period. Figure 3 uses this technique to show the accumulated rainfall associated with the Letter Storms A-M for the 2004 Atlantic Hurricane season.

Figure 3: Accumulated rainfall from Letter Storms A-M for the 2004 Atlantic hurricane season

With the capabilities enabled by spaceborne active and passive microwave instruments, it is not surprising that NASA's operational partners like the National Oceanic and Atmospheric Administration (NOAA) and the Department of Defense (DoD) routinely use TRMM data. The DoD's Joint Typhoon Warning Center regularly relocates storm fixes and adjusts intensity assessments based on TRMM imagery. The TRMM passive microwave imagery is particularly suited for detecting early circulations in storms that may be obscured by more traditional visible or infrared techniques (see Figure 4). NOAA's National Hurricane Center also uses TRMM for such purposes. Both organizations enthusiastically await the extended coverage and better sampling time that GPM will offer.

Figure 4: TRMM-indicated circulation (right) vs. the same storm in the infrared (left) (courtesy of Naval Research Laboratory/DoD)

Understanding Intensification Processes

TRMM's Precipitation Radar (PR)-the world's only spaceborne rainfall radar until GPM launches-provides scientists with data regarding the vertical structure of tropical cyclones. Such information is critical for identifying towering thunderstorms in the eyewall and rainbands of hurricanes. NASA scientists have hypothesized that extremely large thunderstorms or rapid thunderstorm growth in a hurricane may indicate an intensification process. These so-called "hot towers" or convective bursts can most effectively be identified using the "cat-scan" capabilities provided by the spaceborne radar and passive microwave instruments on TRMM. In Figure 5, a TRMM "cat-scan" of 2004's Hurricane Frances reveals a possible hot tower.

Figure 5: Vertical structure of Hurricane Frances (2004) derived from TRMM data

GPM will fly improved dual-frequency Precipitation Radar (PR) on the core satellite along with the GPM Microwave Imager. The TRMM PR has already demonstrated a keen ability to characterize the detailed structure of hurricanes like 2003's Hurricane Isabel. In Figure 6 below, a significant portion of Isabel's lifecycle (from strong category 5 to landfall) is captured by TRMM. This sequence is quite amazing considering the "hit or miss" nature of the TRMM data.

Figure 6: TRMM data shows the intensity of Hurricane Isabel (2003) decreasing as it moves from the open ocean (on right) toward land to the left

A clear relationship between the size of the hurricane eye and intensity is seen: the smaller the eye, the more intense the hurricane. This fact is not surprising since angular momentum and dynamic processes are at play. The same principle can be illustrated by a spinning ice-skater. When she spins with her arms close to her body, her rotational speed increases and vice-versa.

Better hurricane intensification assessment and forecasting are considered landmark achievements; therefore, any additional insight on intensification processes offered by TRMM and GPM will be significant to the research community.

Improving Hurricane Prediction

Global precipitation measurements may also advance research and operational hurricane prediction. The inclusion of TRMM rain rates in numerical models has improved short-term weather and rain prediction. In the post-event model simulation by scientists at NASA Goddard Space Flight Center (shown at left in Figure 7) the track forecast for Hurricane Bonnie (1998) was significantly improved when TRMM rainfall data was assimilated into the model (blue line) as compared to when no rainfall data was included (green line).

Figure 7: Left: Post-event model simulation of track forecast for Hurricane Bonnie (1998) Right: Comparison of forecast track error (y-axis) for model simulations of Hurricane Bonnie when rainfall data is included and excluded from the experiment. (NASA GSFC)

Researchers at Florida State University also showed significant skill in forecasting both track and intensity during the active 2004 season. In the FSU superensemble simulations, TRMM rainfall data was used in the initialization of the model. Reductions in track errors can save $600,000 to $1,000,000 per mile of coast and possibly save lives.

The usefulness of the TRMM mission in the realm of hurricane research bodes well for the success of GPM. With enhanced measurement capabilities, expanded data coverage, and improved data resolution, GPM should greatly facilitate hurricane researchers in their quest to understand this important phenomenon.

By J. Marshall Shepherd

GPM Deputy Project Scientist

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